A SiC (silicon carbide) single crystal has high withstand voltage and high electron mobility. Therefore, the SiC single crystal is used for a semiconductor substrate of a power device. The SiC single crystal is formed by a sublimation method (i.e., a modified Lely method).
In the modified Lely method, raw material of SiC is mounted in a graphite crucible. A seed crystal of SiC is mounted on an inner wall of the crucible in such a manner that the seed crystal faces the raw material. Then, the raw material is heated at a temperature in a range between 2200° C. and 2400° C. so that a sublimation gas is generated. The sublimation gas reaches the seed crystal so that the gas is re-crystallized on the seed crystal. Here, the temperature of the seed crystal is set to be a temperature, which is lower than the raw material by a few tens degrees C. to a few hundreds degrees C. Thus, the SiC single crystal is grown on the seed crystal.
In this modified Lely method, as the SiC single crystal is grown, the SiC raw material is deteriorated, i.e., the composition of the raw material is changed. Thus, the growth amount of the SiC single crystal is limited. Even when additional raw material is added into the crucible during the crystal growth of SiC, the concentration of the sublimation gas is deviated from the initial composition so that the single crystal is not continuously grown with high quality. This is because the sublimation gas of SiC has the composition of silicon and carbon, which is not 1:1, i.e., stoichiometric.
A method for forming a SiC single crystal by a CVD method is disclosed in U.S. Pat. No. 6,030,661 and U.S. Pat. No. 5,704,985. Specifically, the SiC single crystal is grown by an epitaxial growth method. In this method, gas phase raw material is used for forming the SiC single crystal. Therefore, the composition of the raw material is controlled stably. Further, the raw material can be supplied continuously.
However, as the SiC single crystal is grown, environment around the growth surface of the seed crystal is changed so that crystal growth conditions are changed. Accordingly, the quality of the single crystal may be changed in a case where the single crystal is grown longer and larger. Specifically, it is difficult to perform the lengthy crystal growth with high crystal quality.
To improve the above problem, a raising growth method such as a CZ method is disclosed as a prior art. This method is such that the crystal is raised up from melted raw material liquid. The crystal growth is controlled at a boundary between solid and liquid. This technique can be applied to gas phase crystal growth. For example, in Japanese Patent Application Publication No. H06-298594, a crucible is moved down, and a seed crystal is moved up, independently. However, this method is the gas phase crystal growth; and therefore, the raw material gas diffuses everywhere. Thus, a SiC poly crystal is grown at everywhere. Thus, the crystalline of the single crystal is deteriorated, and equipment for manufacturing the SiC single crystal is also damaged.
To limit a raw material gas from diffusing, a method for crystal growth with using a guide is disclosed in Japanese Patent Application Publication No. 2001-226197. In this method, the guide is disposed near the side of the seed crystal so that the raw material gas is prevented from penetrating to the side of the seed crystal. Thus, the crystal growth is performed stably. However, the raw material gas is not completely protected, so that a part of the raw material gas penetrates to the side of the seed crystal. Thus, the diameter of the single crystal becomes larger. Therefore, the single crystal having a target diameter is not obtained.